Not applicable.
Not applicable.
The invention generally relates to oscillometric blood pressure determining techniques, and more particularly to determining the diastolic pressure using that technique.
Knowing the pressures exerted by blood on the blood vessel walls of patients is of great value to those engaged in medical practice. In the case of humans, the pressure in the vascular system is measured for many reasons, including diagnosis, ascertainment of the progress of therapy, the physiological state when under anesthesia, etc. As an example, the determination of arterial blood pressure is an essential element in the diagnosis of a patient suspected of cardiac disease. Normal human arterial blood pressure ranges between 80-120 millimeters of mercury, whereas elevations of arterial blood pressure above that range are found in cases of congestive heart failure, renal artery disease, coarctation of the aorta, etc. Additionally, untreated hypertension is known to be associated with an increased risk of stroke, coronary artery disease, and aneurysms.
During the cycle of the heartbeat, which normally occurs approximately once per second, the arterial blood pressure oscillates. When the heart muscle contracts, known as systole, blood is pushed into the arteries. This increases the arterial pressure. When the heart muscle relaxes, known as diastole, the arterial blood pressure falls. The maximum of the arterial pressure oscillation during the heartbeat is known as systolic pressure; the minimum is known as diastolic pressure. The arterial pressure versus time waveform can also be used to calculate what is known as mean arterial pressure. The mean arterial pressure (MAP) is calculated by integrating the arterial pressure waveform for one cycle and then dividing that quantity by the cycle period. The indirect techniques of oscillometry and auscultation are used in practice to estimate the systolic, mean, and diastolic pressures non-invasively. However, it is known that under certain rare conditions the diastolic estimate which oscillometry produces is inaccurate, yet the systolic and MAP estimates are good. It is the purpose of this invention to improve the diastolic estimate using easily obtained, but previously ignored oscillometric information.
The auscultatory method is commonly used by medical personnel to indirectly measure arterial blood pressure. In this technique, constrictive pressure is gradually applied about the limb of the patient until the flow of blood through the limb vessel has been arrested, as determined by listening to a stethoscope applied over the vessel at a point distal the point of constriction. Then upon gradual release of the constriction pressure, the beginning of the flow through the vessel can be heard and the constriction pressure is noted on a gauge reading in millimeters of mercury. This pressure is referred to as systolic pressure and is taken as an estimate of the true intra-arterial systolic pressure. The pressure then is gradually released further until the sounds of the flow again cease and the pressure is again noted, which pressure is referred to as diastolic pressure and is taken as an estimate of the true intra-arterial diastolic pressure. The difference between the diastolic pressure and systolic pressure is termed pulse pressure. Previously the constriction pressure has been derived from an inflatable cuff connected to a mercury column manometer or to an aneroid type gauge having a dial scale calibrated in millimeters of mercury. It is also known that the auscultatory estimate of diastolic pressure can at times be inaccurate; auscultation can be very technique dependent and varies, for example, due to the hearing ability of the clinician taking the reading. Furthermore, auscultation can, in some cases, be quite confusing when determining diastolic estimates because the Korotkoff sounds may never disappear as the cuff pressure is lowered.
A previous automatic indirect blood pressure reading apparatus employed the oscillometric method in which an arm cuff is inflated to a pressure at which blood flow is occluded. The cuff then is deflated at predetermined pressure increments in a step-wise manner. At each step, the pressure in the cuff is measured repeatedly using a suitably short sampling period in order to detect pressure fluctuations. The instantaneous pressure in the cuff is due to the inflation pressure and the force exerted by the pressure pulsations in the patient""s blood vessels during each heartbeat. The beating heart causes the pressure in the cuff to oscillate at each step of deflation. The apparatus continues in this fashion until a complete envelope of oscillation amplitude versus cuff pressure is obtained. The cuff pressure at which the maximum amplitude oscillations are obtained is indicative of the mean arterial pressure. The systolic and diastolic pressure estimates are also determined from predefined functions of the envelope data. The oscillometrically determined systolic, MAP, and diastolic are considered estimates of the true intra-arterial pressure values. However, it is also known that arterial compliance plays a major role in the estimating functions; arterial compliance can change in complicated and unpredictable ways as physiological circumstances change.
The oscillometric blood pressure is determined indirectly from a cuff that is placed around a portion of the body, such as an upper arm, of the human being whose blood pressure is desired. The cuff is inflated to a predetermined pressure, preferably great enough to occlude the flow of blood in the limb of the patient. Then the cuff is deflated in a controlled manner to produce a deflation pressure in the cuff that decreases with time. In the preferred embodiment, the cuff is deflated in regular pressure increments thereby producing a plurality of discrete deflation pressure levels.
During each of a plurality of heartbeats, the pressure oscillations that occur at the discrete deflation pressure levels are measured and stored in the apparatus. The complete data set of the amplitude of the oscillations versus the discrete pressure levels is known as the oscillometric envelope. The oscillometric estimate of the mean arterial pressure is determined from this envelope data. For example, the estimate of the mean arterial pressure (MAP) is the deflation pressure level that occurs when the oscillation measurements have the greatest amplitude. Similarly, the systolic pressure can be estimated from the envelope data by finding the discrete pressure level which occurred when the oscillation amplitude is a predetermined fraction of the maximum oscillation size at cuff pressures above MAP. Note that interpolating between discrete deflation pressure levels may produce a more accurate estimate of systolic pressure.
Diastolic pressure can be estimated from the envelope data by finding the discrete pressure level which occurred when the oscillation amplitude is a predetermined fraction of the maximum oscillation size at cuff pressures below MAP. Interpolating between discrete deflation pressure levels may produce a more accurate estimate of diastolic pressure. This method can lead to errors in the determination of diastolic pressure under some circumstances.
In the preferred embodiment, the diastolic pressure is determined by measuring the area of the oscillation complexes. The diastolic pressure is determined by finding the deflation pressure below MAP that produces the largest oscillation area.
If for a given measurement, the measured amplitude under the oscillation pulse is greatest at MAP, then the diastolic pressure will be determined from the deflation pressure where the oscillation amplitude is a predetermined fraction of the maximum amplitude.
In an alternative embodiment, the diastolic pressure is determined by finding a first deflation pressure at which the maximum oscillation area occurs and a second deflation pressure at which the predetermined amplitude ratio occurs. The diastolic pressure is calculated as the average of the first and second deflation pressures.